1. Field of the Invention
The present invention relates to a honeycomb formed body extruding die. More particularly, it relates to a honeycomb formed body extruding die which is for use in extrusion of a honeycomb formed body and is capable of inhibiting deformation of cell partition walls of the honeycomb formed body.
2. Description of the Related Art
Heretofore, in a manufacturing method of manufacturing a honeycomb structure made of a ceramic material, an extruding die of the honeycomb formed body (hereinafter simply referred to as “the die”) has been used, and there has broadly been performed an extruding method of extruding a forming material (a kneaded material) from the die. Here, the die generally has a lattice-shaped slits disposed in one surface of a die substrate and having a width corresponding to a partition wall thickness of cell partition walls of the honeycomb formed body which becomes a forming object, and back holes formed in the opposite surface of the die substrate to communicate with the slits. The back holes are disposed at positions corresponding to slit intersecting portions in which the lattice-shaped slits intersect with one another, and in the die substrate, the back holes communicate with the slits in the slit intersecting portions. In consequence, the forming material introduced from the back holes on a die backside into the die substrate is discharged from the back holes toward a die front side formed with the slits. Further, there is extruded the honeycomb formed body having a plurality of cells defined by cell partition walls corresponding to a slit shape and having a rectangular or hexagonal shape. The manufacturing of the honeycomb structure includes kneaded material preparation, extrusion, raw cutting, drying, finish-cutting and firing steps. Here, the material from the extrusion before firing is referred to as the honeycomb formed body, and the fired honeycomb formed body is referred to as the honeycomb structure.
The die is constituted of a metal block of a stainless alloy, cemented carbide or the like, a plurality of back holes are bored in the metal block on its one surface side by drill processing or the like, and slits communicating with the back holes in the slit intersecting portions from the other surface side of the metal block are formed by using a well-known metal processing technology such as grind processing or discharge processing (see Patent Document 1).
In a case where the extrusion of the honeycomb formed body is performed by using this die, when a local deviation is present in the dimension or surface roughness of each portion of the die, a local deviation of a flow resistance occurs to the forming material, and a difference is locally made in extruding speed to a cross sectional direction of the honeycomb formed body. Due to this difference in extruding speed, the cell partition walls are deformed when the material is extruded from the die, and it might become difficult to form the honeycomb formed body having a suitable cell shape. For example, there might occur a deformation in which lattice-shaped cell partition walls defining the respective cells of the honeycomb formed body wind along a cell longitudinal direction (matching an extruding direction), a deformation in which the cell partition walls wind along an orthogonal direction (matching the above cross sectional direction) to the cell longitudinal direction, and a deformation in which these deformations are combined. Such a deformation occurs especially remarkably in the honeycomb formed body in which an open frontal area of an end face is large (e.g., 85% or more), i.e., the honeycomb formed body in which a partition wall thickness of the cell partition walls having a larger cell area than the end face is small. The above cell deformations, not only deteriorate a mechanical strength of the honeycomb structure, but also bring clogging of catalyst coating or increase of passed air resistance in loading a catalyst, when the honeycomb structure is used as a catalyst carrier.
Concerning such a defect of the cell shape of the honeycomb formed body, a fluidity in the die is locally regulated, and the extruding speed (a flow speed) of the forming material to be extruded through the slits is regulated in a predetermined range, whereby the occurrence of the deformation of the cell shape in the honeycomb formed body can be decreased. In consequence, the honeycomb formed body having a stabilized quality can continuously be formed.
On the other hand, it is known that corner portions of each of polygonal cell blocks defined by the slits formed on an extrusion side of the die are rounded or chamfered, whereby roundness is provided in each cell corner portion of the honeycomb formed body to be extruded, a thickness of a catalyst coating layer to be loaded onto the honeycomb structure is uniformed, and there is inhibited generation of disadvantages such as cracks or breaks due to concentration of stress to be applied to the corresponding portion when the honeycomb structure is dried and fired (see Patent Documents 2 and 3).
[Patent Document 1] JP-A-H11-138354
[Patent Document 2] JP-B-S61-20403
[Patent Document 3] JP-A-H08-169005